The invention relates to motor drives in general, and more particularly to a motor drive wherein an adaptive system associated with vector control is used concurrently with instantaneous identification of the speed parameter.
Variable speed induction motor drives using static inverters are widely used. When dynamic performance is not important, the speed of the motor drive is simply adjusted by changing the frequency at the input while keeping the volt-hertz ratio constant. There are situations, however, requiring fast response, for instance in servo-applications. Then, feedback from speed information from the rotating shaft of the machine is usually required. Such feedback is part of a closed loop for control, or it may be part of the torque control algorithm for the determination of the slip frequency of the motor excitation. In such instances, typically, a tachometer, or a high resolution encoder, is mounted on the motor shaft in order to provide such speed feedback.
The present invention is directed to speed control of an induction motor drive with dynamic response, but without the use of any feedback transducer on the motor shaft. High performance speed control where it is uneconomical, or impractical, to have a shaft-mounted speed transducer, is desirable. This is the case with linear-motor-driven transportation systems, or with steel mill drives because transducer cabling is undesirable in an environment which is inhospitable around the motor. It is also the case with retrofit applications where an existing induction motor having no shaft transducer installed, needs to be speed-controlled. A tacho-less speed measurement is also useful for induction motor control and protection applications. Speed information derivation is particularly useful for "intelligent" motor protection, whatever the nature and performance of the drive.
Tacho-less induction motor control is known. See for instance: U.S. Pat. No. 4,009,427 of Shuichi Takahashi and U.S. Pat. Nos. 4,503,376 and 4,680,526 both of Toshiaki Okuyama.
The invention involves estimating the speed of an induction motor with good dynamic response and accuracy as if it were a rapidly changing parameter of the system. To this effect, recourse has been made to prior art methods of parameter estimation, using Model Reference Adaptive Control.
Model Reference Adaptive Control has been described "Adaptive Control--The Model Reference Approach" by Yoan D. Landau, published by Marcel Dekker, Inc. N.Y. 1979. This approach involves two models of the subject system, both having the same mathematical structure, both being fed with the same inputs. One of the models called the "reference model", is fixed as regards its parameters, while the other has variable parameters and is called the "adjustable model". An "adaptation mechanism" is used to dynamically adjust the parameters of the adjustable model until operation thereof coincides with the operation of the reference model. Control of the adaptation mechanism to this effect results from signals derived from the two models which represent the instantaneous difference between the state variables, or the outputs of the two models. The Landau publication is incorporated by reference.
It is known to identify the speed of an inverter-fed induction motor by the technique of Model Reference Adaptive Control. See for instance: "Speed Sensor-less Vector Control of Induction Motor with Model Reference Adaptive System" by Shinzo Tamai, Hidehiko Sugimoto and Masao Yano on pages 189 to 195, a paper presented at a IEEE Conference in Atlanta, Ga. on 18-23 Oct. 1987 IA Vol. 1. This article is hereby incorporated by reference.
Other articles of interest are: (1) "Observers for Flux Estimation in Induction Machines" by George C. Verghese and Seth R. Sanders IEEE Trans. Industrial Electronics, Vol. 35, No. 1, February 1988, pp. 85-94; 2) Electrical Engineering in Japan, Vol. 107, No. 4, 1987 "Vector Control System for Induction Motor Using a Speed Estimation Based on Instantaneous Slip Frequency Principles" by Hirotami Nakano, Schinichi Horie, Tsuyoshi Matsuo and Kohji Iwata, pp. 95-103.
U.S. Pat. Nos. 4,009,427 and 4,733,149 relate to adaptive control of an induction motor.